Cell encapsulation is a promising strategy for immunoisolating single cells and cell clusters and thus preventing any immune response that would compromise the functionality of the cells upon implantation. Bio-encapsulation has been extensively employed for novel therapeutic trials in the fields of diabetes, hemophilia, cancer and renal failure. However, most trials have not been fully successful for a combination of reasons:                lack of reproducibility in encapsulation and cell isolation methods;        lack of suitable encapsulation materials which should be biocompatible, mechanically and chemically stable, and have an appropriate pore cut-off size to allow nutrient and by-product flow in and out of the capsule while protecting encapsulated biomaterial from immune system effects;        production of non-uniform or non-conformally coated capsules (affecting oxygen and nutrient diffusion through the capsule and therefore encapsulated cell viability);        inability to scale up the encapsulation process from small animal studies to pre-clinical non-human primate studies; and        choices of unfavorable transplantation sites.Such challenges to encapsulation technology may be seen in the context of work in one of the most promising therapeutic fields for cell encapsulation: diabetes.        
Diabetes results from the autoimmune destruction of pancreatic beta cells, one of the several cell types which make up the islets of Langherans. Over the course of their lifetimes, diabetic patients must frequently monitor and control blood glucose levels and administer insulin when they experience hyperglycemia, which has many collateral effects. Islet allo-transplantation is a very promising therapy to treat diabetic patients, but requires a lifetime of systemic immunosuppression to avoid allograft rejection.1 
To avoid administration of immunosuppressive drugs at the systemic level, islet allografts can be immunoprotected by coating the cells for transplantation with a polymeric capsule that allows diffusion of oxygen, glucose and insulin while preventing cell-cell contact and diffusion of cytotoxic molecules, which otherwise would trigger the immune response against the graft and its ultimate rejection by the host.2 Islets have a non-uniform size that varies from about 50 to 300 μm in diameter. Most coating procedures developed by others do not allow conformal coating of islets; capsule diameter is generally constant and independent of islet size, and is thus normally larger than 300 μm to guarantee coating of larger islets.3 Because of the excess of cell-free coating material, the total volume of the islet implant is greatly increased such that the only appropriately-sized grafting site is the poorly-oxygenized abdominal cavity, which contributes to hypoxia of the encapsulated cells. Further, the thickness of the capsule increases the diffusion barrier to oxygen through the coating, also aggravating cell hypoxia, and delays glucose sensing and thus responsiveness of insulin secretion4 (FIG. 1A). Most of these encapsulation methods are based on generation of droplets of the coating material mixed with islets through air-jet pump or electrostatic droplet generators.5 
In contrast with encapsulation methods based on droplet generation, conformal coating of cell clusters of various diameters has been the focus of some recent investigations. Most of these methods are based on either (a) coating formation layer-by-layer directly onto cells (e.g., by chemical reaction or photo-polymerization) or (b) a purely hydrodynamic procedure, typically involving formation of particles by water in oil emulsion formation or by break-up of a water jet in oil by the fluid dynamic principle of Rayleigh-Plateau instability.3,10 Using these methods, it is possible to generate water particles with a constant diameter uniquely dependent on the characteristics of the water and the oil phase, the surface tension between the two phases and the ratio of the hydrodynamic parameters of the two phases.6,7,8,9 In the food and pharmaceutical industries, these methods have been extensively exploited to nano-encapsulate water-soluble drugs and other substances8 and have only recently been extended to encapsulation of micron-size single cells and cell clusters, with some reported success, as described below.
Chabert M. and co-workers developed a microfluidic high-throughput system for encapsulating and self-sorting single cells based on the principle described above.10 However, their system is designed for encapsulation and sorting of single cells (40 μm in diameter or less), and cannot be applied to cell clusters because of the micro-dimensions of their device, which would subject non-single cells to unaffordable shear stresses.
Garfinkel M. R. and co-workers developed another method to encapsulate islets by selectively withdrawing the islet-water phase from an external oil phase to create a thin coating on cell clusters. In this method, water phase jetting in the oil phase is achieved by suction of the water phase layer on top of the oil phase. In this design, turbulent flow is created in the water withdrawal area, ultimately leading to incomplete coating that necessitates a second round of encapsulation, increasing the amount of stress to which the cells are subjected and reducing the yield of the process.11 Further, the gel polymerization is achieved through photo-polymerization, which may compromise long-term function of the coated cells.
Hubbell J. A. and co-workers developed an approach of coating by a chemical reaction directly on the cell surface, whereby a photosensitizer was adsorbed to the surface of islets, and the photosensitizer-treated islets were suspended in an aqueous solution of a photopolymerizable macromer (U.S. Pat. No. 6,911,227). Photoillumination of the islet suspension led to the polymerization and crosslinking of the macromer to create a conformal polymer gel bound to the surface of the islets.
In view of the above, there remains a need in the art for efficient, high-yield methods of conformally coating cells and cell clusters without compromising cell functionality.